I would also suggest trying another plumber. Getting the dip tube out involves disconnecting the cold-side plumbing and removing the nipple, after which it is simple to remove the tube, as it merely sits on a metal ring in the port. Even if you have hard copper plumbing, cutting it and resoldering is going to be a lot cheaper than a new heater. I don’t understand why the first plumber didn’t do just that.
The details can be found under Know-how, which is under Preventive maintenance.